Certain furnaces, typically referred to as fired heaters, heat a hydrocarbon to a specific temperature to allow for a subsequent processing step. A fired heater includes a fire box, a stack, burners, radiant section tubes or pipes, and convective section tubes or pipes. The fire box is a refractory lined steel structure which contains the burners, radiant section tubes and convective section tubes. The stack is a steel conduit that carries the combustion gases out of the fire box to the atmosphere. The burners ignite a fuel and air mixture to generate the required heat.
The radiant section tubes line the inside wall of the fire box. These tubes are connected to each other with 180 degree return bends, resulting in a serpentine coil. In order to maximize the number of tubes that will fit into the firebox, these return bends typically have a radius equal to one tube diameter. The convective section tubes are located at the top of firebox below the stack. The combustion gases pass over these tubes as they exit the firebox and enter the stack. To maximize the transfer of heat out of the combustion gases and into the hydrocarbon the convective section tubes have extended surfaces on the outside surfaces. The convective section tubes usually are connected in the same manner as the radiant section tubes. However, the convective section tubes are arranged in a "stacked" manner, resulting in typically 3 to 5 rows of tubes staggered one row on top of the other.
Fired heaters employed in the oil refining and chemical industry experience elevated temperature and potentially corrosive conditions. These conditions result in deterioration of the tubes. If such deterioration goes undetected and uncorrected the tubes will rupture during operation. Such a rupture will result in the release of a hydrocarbon into the firebox. This release will result in a fire in the firebox and will require that the fired heater be shutdown for repair. This unscheduled shutdown can often result in the shutdown of an entire operating unit and in some cases an entire plant.
A method to easily and rapidly inspect the tubes in a fired heater would allow the operator to monitor tube deterioration and replace tubes before any such failure. Currently, most fired heaters are inspected during a shutdown by manually measuring the wall thickness of the tubes using an ultrasonic thickness gauge. The radiant section tubes typically are measured at several points along their lengths. These readings are recorded by an inspector. Some owners of fired heaters employ supplemental inspection techniques including x-ray and tube gauging.
Inspecting the tubes in a fired heater is difficult and time consuming. The entire tube set is enclosed in the firebox making access difficult. The radiant section tubes are accessible for limited manual inspection. This access usually requires the construction of scaffolding in the firebox. Constructing this scaffolding is both time consuming and costly. The close proximity of the radiant section tubes to the firebox wall limits any manual inspection to the tube surfaces not facing the wall. The manual nature of the inspection requires that in the interest of time and cost that the number of inspection points be limited. The convective section tubes are for the most part not accessible for manual inspection. The stacked configuration makes the center rows inaccessible. The extended surfaces on the outside of the convective tubes limits manual inspection to visual observations.
The extremely short radius of the return bends make the use of conventional pipeline inspection tools difficult or impossible for fired heater use. Additionally, the large number of return bends in a section of tubes makes the use of inspection tools with a connecting cable or tether impossible beyond a certain number of bends.
The problem with bends and cabled or tethered devices is described in U.S. Pat. No. 4,050,384 (Chapman) and may be referred to as the capstan effect. As the cable goes around a bend the frictional forces increase geometrically with each bend. After a few bends the cable drags and generates so much force that the cable would break.
U.S. Pat. No. 4,050,384 describes an approach to overcome the capstan effect by placing "traction units" along the cable. While this method will overcome the capstan effects it is plagued with the practical problems of introducing such an apparatus into a conduit system with a fluid.
U.S. Pat. No. 3,603,264 (Von Arx) describes a typical tethered inspection tractor. The problem with this system is the drag on the cable as it pass down the pipe. This device would have difficulty going up a vertical pipe. U.S. Pat. No. 4,601,204 (Fournot et al.) describes a self displacement tractor that is capable of running up a vertical pipe. One disadvantage of this type of device is that it is still tethered and therefore restricted in its length and ability to handle multiple bends. Another disadvantage is that the advance of these types of devices are usually quite slow and is measured in feet per minute not feet per second as would be desirable.
There are a number of conduit inspection devices that have short U-joint connections linking two or more modules. Other devices have a rigid shaft with a ball on each end that mate within module sockets such as disclosed in U.S. Pat. No. 4,852,391 (Ruch et al.). All of these devices have certain limitations, especially for negotiation of tight radius bends.